Most electrical conductors used in electronic devices are made of metals, such as copper, aluminum, gold, silver, lead/tin (solder), molybdenum and others. Solder connection technology using lead/tin alloys plays a key role in various levels of electronic packaging, such as flip-chip connection (or C4), solder-ball connection in ball-grid-arrays (BGA), and IC package assembly to a printed circuit board (PCB). Solder joints produced in the electronic packages serve critically as electrical interconnections as well as mechanical/physical connections. When either of the functions is not achieved, the solder joint is considered to have failed, which can often threaten a shut-down of the whole electronic system.
When microelectronic packages are assembled to a printed circuit board, the lead-tin eutectic solder, 63% Sn-37% Pb, having the lowest melting point (183.degree. C.) among Pb--Sn alloys, is most widely used. In these applications, there are two solder connection technologies employed for mass production: plated-through-hole (PTH) and surface mount technology (SMT) soldering. The basic difference between the two technologies originates from the difference in the PCB design and its interconnection scheme.
In SMT soldering, microelectronic packages are directly attached to the surface of a PCB. A major advantage of SMT is high packaging density, which is realized by eliminating most PTH's in the PCB as well as by utilizing both surfaces of the PCB to accommodate components. In addition, SMT packages have a finer lead pitch and a smaller package size compared to traditional PTH packages. Hence, SMT has contributed significantly in reducing the size of electronic packages and thereby the volume of the overall system.
In SMT soldering, solder paste is applied to a PCB by screen printing. Solder paste consists of fine solder powder, flux, and organic vehicles. During the reflow process, solder particles are melted, flux is activated, solvent materials are evaporated, and simultaneously molten solder coalesces and is eventually solidified. In contrast, in the wave soldering process, a PCB is first fluxed and components are mounted on it. Then it is moved over a wave of molten solder.
The soldering process is usually completed by subjecting the solder joints to a cleaning step to remove residual flux materials. Due to environmental concerns, CFCs (chlorofluoro carbons) and other harmful cleaning agents used for this purpose are being eliminated and water-soluble or no-clean flux materials are being used to minimize or eliminate the cleaning steps.
Recent advances in microelectronic devices demand a very fine pitch connection between electronic packages and a printed circuit board (in an order of a few hundred micrometer pitch). The current solder paste technology used in SMT can not handle this very fine pitch interconnection due to the soldering defects such as bridging or solder balling. Another technical limitation of using the Pb--Sn eutectic solder is its high reflow temperature, approximately 215.degree. C. This temperature is already higher than the glass transition temperature of the epoxy resin used in most polymeric printed circuit board materials. Thermal exposure at this reflow temperature produces significant thermal strains in a printed circuit board after soldering, especially in the direction perpendicular to the surface of a PCB, because no structural reinforcement is made in that direction. Thereby, the residual thermal strains in an assembled PCB could significantly degrade the reliability of an electronic system.
A more serious concern regarding the usage of lead (Pb)-containing solders is an environmental issue, a trend already experienced in other industries and has led to the elimination of lead from gasoline and paints.
In the electronic industry, two different groups of materials are investigated currently for the possibility of substituting the Pb-containing solder materials; Pb-free solder alloys, and electrically conductive pastes (ECP). The present invention discusses the development and applications of the electrically conductive paste materials. An electrically conductive paste (or adhesive) is made of metallic filler particles loaded in the matrix of a polymer material. The polymer matrix can be any polymer suitable for use in a paste, for example, a thermoplastic of thermoset. The polymer is selected preferably from the group comprising epoxy, polyester and polyimide. The soluble epoxy, in particular, soluble ketal and a acetal diepoxides, as described in U.S. application Ser. No. 08/210,879, the teaching of which is incorporated herein by reference can also be used as the polymer matrix. Referring to FIG. 1, silver-particle 4 filled epoxy 6 is the most common example of the electrically conductive pastes 2, schematically shown therein as disposed between surface 8 and surface 10. The silver particles usually in the shape of flakes provide electrical conduction by percolation mechanism, while the epoxy matrix provides adhesive bond between the components and a substrate. This silver-filled epoxy material has been long used in the electronic applications as a die-bonding material, where its good thermal conduction rather than electrical conduction property is utilized. However, this material has not been accepted for the applications requiring high electroconduction and fine pitch connection. The silver-filled epoxy material has several limitations, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, silver migration, difficulty in rework, and others. Since this silver-filled epoxy material is electrically conductive in all the directions, it is classified as "isotropic" in electro-conduction. There is another class of electrically conductive adhesive (or film), which provides electroconduction only in one direction. This class of the materials is known as "anisotropic" conductive adhesive film 12, shown schematically in FIG. 2A, which contains electrically conductive particles 18 in a binder or adhesive material 20. The anisotropic conductive adhesive or film 12 becomes conductive only when it is compressed between two conducting surfaces 22 and 24 as shown in FIG. 2B. This process normally requires heat and pressure. The major application of the anisotropic conductive film is for joining of a liquid crystal display panel to its electronic printed circuit board. The conducting particles 18 are usually deformable, such as solder balls, or plastic balls coated with nickel and gold. The binder or adhesive material 20 is mostly a thermosetting resin.
The ECP made of Sn-plated Cu powder and polyimide-siloxane resin disclosed in our earlier U.S. patent application Ser. No. 08/641,406 filed on May 1, 1996, the teaching of which is incorporated herein by reference, is a good candidate for the high temperature solder joints such as controlled collapse chip connections (C4) and solder ball connection (SBC) to a ceramic substrate. However, for the polymeric printed circuit board applications, this ECP is not adequate, because the reflow temperature such as 250.degree. C. is much higher than the glass transition temperature of the polymeric resin, for example, FR-4. Candidates for this purpose are ECP's made of Cu powder plated with indium, tin-bismuth alloys or indium-tin alloys, formulated with polyimide-siloxane resin. The reflow temperature of these powder pastes is expected to be between 120 and 180.degree. C., which is even lower than the reflow temperature of the Pb/Sn eutectic solder, 215.degree. C.
In an earlier U.S. patent application Ser. No. 08/414,063 filed on Mar. 31, 1995, the teaching of which is incorporated herein by reference, we have disclosed a process to produce dendritic copper powder overcoated with Sn or Sn and BiSn coatings by electrolytic plating on a rigid inert cathode. The morphology of the powder that can be made by this technique is restricted to the dendritic shape which is not always the preferred one for all ECP applications.
A solder/polymer composite paste material is disclosed in U.S. Pat. No. 5,062,896 (Huang et. al.), comprising a major proportion of a meltable solder powder filler, such as Bi--Sn, Pb--Sn, Bi--Sn--Pb alloys, a minor proportion of a thermoplastic polymer such as a polyimide siloxane, and a minor proportion of a fluxing agent. An oxide-free, partially coalesced solder alloy connection is obtained, which is polymer strengthened and reworkable at a low reflow temperature, per se, or in the presence of polymer solvent.
In U.S. Pat. No. 5,286,417 (Mahmoud et. al.), a fusible conductive adhesive is disclosed, which comprises metal alloy fillers such as Sn--Au and Bi--Au, and a thermoplastic polymer having a glass transition temperature overlapping the melting temperature of the metal filler alloys. The loading of the conductive material in the polymer is in the range of about 15% to about 20% by weight.
In U.S. Pat. No. 5,136,365 (Pennisi et. al.), an adhesive material is disclosed, which contains a fluxing agent and metal particles for use in reflow soldering such as Sn, Pb, In, Bi, Sb, Ag and others, in the matrix of an epoxy resin. Upon reflow soldering, the said adhesive forms anisotropic electroconduction between an electrical component and a substrate.
In U.S. Pat. No. 5,213,715 (Patterson et. al.), a directionally conductive polymer is disclosed, which contains a metallic filler powder of Ni or Cu. The metallic powder is treated by a different polymer than the polymer used as a matrix resin. Upon compression, the coated polymer dissolves to make an electrical conduction among the filler particles.